Towns of some sophistication are built from the fifth millennium BC by people practising agriculture on the banks of the Indus. They shelter within protective walls; they have drainage systems, and an oven within each mud-brick house. By 3200 BC there are settlements of this kind along the length of the river.

In about 2500 BC the river becomes the lifeline of a much more highly developed civilization, based on two places which are unmistakably cities - Harappa and Mohenjo-daro. These cities, and their civilization, vanish without trace from history until discovered in the 1920s.

Life in the Indus valley cities seems to have been highly regulated. Streets are laid out on a rectangular grid pattern, and there is a sewage system with household drains leading into main sewers of baked brick. These even have inspection holes for maintenance.

The larger houses, of two or occasionally three storeys, show blank walls to the outer world but have an inner courtyard - possibly with wooden balconies giving onto it.

The public buildings of these cities also suggest a high degree of social organization. The great granary at Mohenjo-daro is designed with bays to receive carts delivering crops from the countryside, and there are ducts for air to circulate beneath the stored grain to dry it. The granary at Harappa has a series of working platforms close to barrack-like dwellings, suggesting that workers live here (very possibly government slaves) and that they grind corn on the platforms for the city's supply of bread.

At Mohenjo-daro, close to the granary, there is a building similarly civic in nature - a great public bath house, with steps down to a brick-lined pool in a colonnaded courtyard.

The seals of the Indus valley: from 2500 BC

As in the other great early civilizations, the bureaucrats of the Indus valley have the benefit of writing to help them in their administration. The Indus script, which has not yet been deciphered, is known from thousands of seals, carved in steatite or soapstone.

Usually the centre of each seal is occupied by a realistic depiction of an animal, with above it a short line of formal symbols. The lack of longer inscriptions or texts suggests that this script is probably limited to trading and accountancy purposes, with the signs establishing quantities and ownership of a commodity.

Cotton, rice and sesame: 2500-1700 BC

The local produce of the Indus civilization includes three crops of great significance in subsequent history, each of which is possibly first cultivated here.

Yarns of spun cotton have been found at Mohenjo-daro. There is evidence of the growing of rice in the region of Lothal. And sesame, the earliest plant to be used as a source of edible oil, also seems to make its first appearance here as an agricultural crop. Engravings of elephants on the Indus valley seals, sometimes with ropes around the body, suggests that this civilization is also the first to tame the world's most powerful beast of burden.

Peak and decline: 2000 - 1700 BC

The reach of the Indus civilization is extensive. After the discovery of Harappa and Mohenjo-daro, further sites have been revealed - as far down the coast as Lothal, making the spread of the Indus civilization greater than that of Egypt and Mesopotamia together.

At Lothal there is even a specially designed dockyard, of kiln-baked bricks, from which vessels trade along the coast and possibly up the Persian Gulf as far as Mesopotamia.

The sense of order, so evident in the Indus cities, begins to diminish after about 1900 BC. Less imposing buildings, of more flimsy construction, are inhabited now by a declining population. Many reasons have been suggested - an impoverished agricultural base due to over-exploitation, or a succession of devastating floods. The discovery of several unburied bodies in a street in Harappa has led to suggestions of a sudden and violent end.

Certainly the Indus civilization is followed by a violent intrusion into northwest India, that of the Aryans. But they do not arrive until about 1500 BC. The cities of the Indus seem to have declined before then into their long spell of invisibility.